204 



KNOWLEDGE. 



[September 1, 1899. 



cloud of the day, coming after a spell of very fine weather. 

 Borne by a north-easterly wind, it was gradually drifting 



Fia. 5.— Bain-ClouiU, 1898, May, 12a. 3h. 30m. 



to the south-west. Other cirri then followed, 

 while a few cumuli put in an appearance 

 towards four o'clock in the afternoon. The 

 sky became finally overcast, and 0'13 in. of 

 rain fell on the morrow. 



In Fig. 1 of the accompanying Plate we 

 have a fine cirro-cumulus texture, with clouds 

 at the lower level (stratus) crossing the sun, 

 as if " brushed up " from the south-west. 



A coarser cirro-cumulus is shown in Fig. 2. 

 Here we note heavier cumuliform masses, 

 bordering on what is termed AUo-Cumidus on 

 the Continent. In the upper part of the 

 photograph, the small clouds are separated 

 along curious, definite, and nearly parallel 

 straight lines. The formation of these cirro- 

 cumuli was preceded and followed by fine 

 weather, though it coincided with a slight 

 barometric depression. 



Fig. 3 is a view of a bank of stratus above 

 the valley of the Seine, east of Juvisy. These 

 clouds had given rise to foggy, overcast 

 weather a few days previously. On the 



But Mr. Scott, of the Meteorological Office, says that stratus 

 cloud is " not in any way consolidated into a definite 

 form."* 



The cumuli of Fig. 4 are of those peaceful 

 little clouds, following, accompanying, and 

 forecasting fine, warm, summer weather. 

 The perspective grouping of the clouds near 

 the horizon, to the right, shows an appearance 

 of RolUrumulua, while the upper cloud, passing 

 in the vicinity of the sun, shows a dark body 

 with golden fringes. 



One is struck by the flatness of the base of 

 the cumulus. Such horizontality is due to 

 the fact that the base marks the height above 

 the earth's surface, at which the ascending 

 vapours, chilled by expansion, reach the dew- 

 point. Another interesting feature about 

 cumuli is their remarkable appearance of 

 solidity ; — " The hills and white valleys at our 

 feet," says M. Flammarion, in one of his 

 balloon ascents, "appear so solid as to invite 

 us to leave the car and step on them. What 

 a surprise, were we to yield to this tempta- 

 tion ! " 



But cumuli do not always assume this 

 serene, benignant appearance. They easily 

 swell into menacing mountains, with silvery 



Fig. 7.- 



Fig. 6. — Scud di-iftinj; before a grey sheet o£ Cirro-Stratiis, 

 1898, July, 19d. Sh. 50m. 



photograph, contrast is deficient, and the outlines are vague. 



-A Photograpli of the Rainbow, taken at Juvisy after a violent 

 storm, on 1898, June, I'd. 5h. 20m. local time. 



white crests, and sombre, thundering bases. This is 

 particularly the case during a thunderstorm (Fig. 1 

 of the accompanying Plate), where the proud masses, 

 scaffolded by an uninterrupted supply of vapour from 

 below, roll confusedly over each other, heaping themselves 

 into pyramidal figures of fantastic appearance. 



The nimbus of Fig. 5 was taken during a showery day 

 of the spring of 1898. Rain is falUng on the horizon, 

 towards the right-hand side of the picture. It should be 

 remembered that it is to the successive reflections and 

 consequent exhaustion of the light falling ou the water 

 droplets that rain-clouds owe their darkness, and that the 

 light is scarcely quenched here by true absorption. 



"Light scud-clouds driving across heavy masses,' says 

 Admiral Fitzroy, " show wind and rain," Such are the 

 small clouds of Fig. 6, drifting in the foregroimd of 



* "Instructions in the Use of Meteorological Instruments," 1888, 

 p. 67. 



